The major obstacle for obesity treatment is not producing weight loss but in maintaining weight losses over time. Most previous research has been aimed at understanding the factors involved in weight loss and comparing different interventions for weight loss. We propose that weight loss and weight loss maintenance are different processes and that some strategies that work well for losing weight do not work well for keeping weight off. Specifically, we propose that the strategy used by most dieters to keep weight off - food restriction - is not likely to lead to success in keeping weight off. We believe that the body has a physiological drive to maintain a high level of energy expenditure (i.e. high energy throughput). The high energy expenditure can be obtained by maintaining a high level of physical activity or by increasing body weight. Obese individuals have achieved a high level of energy expenditure in part through the process of becoming obese. After weight loss, total energy expenditure is reduced due to declines in metabolism. We have termed this reduction as the "energy gap" and filling the "energy gap" is the challenge for weight loss maintenance. We believe that most people try to fill the "energy gap" by restricting energy intake. This is difficult over the long-term because it opposes both biology and the environment. Our previous data from the National Weight Control Registry suggest that those individuals who are successful in long-term weight loss maintenance close the "energy gap" by physical activity, thus allowing energy balance to be achieved at high energy expenditure (i.e. high energy throughput). We propose to evaluate this hypothesis by randomly assigning individuals to different methods for filling the "energy gap" following weight loss. We hypothesize that those who are assigned to close the "energy gap" with physical activity will regain less weight than those assigned to close the "energy gap" with food restriction alone or with a combination of increased physical activity and food restriction. We further propose to use leading edge techniques to assess how a high energy throughput produced by a high level of activity affects components of energy balance and matching of intake with expenditure. If our hypothesis is correct, estimating the "energy gap" could be done in clinical practice and could provide a quantitative prescription for lifestyle changes required for weight loss maintenance. It would provide a new strategy to help the public be more successful at maintaining a weight loss.